Applied Physics Express 10, 012201 (2017)https://doi.org/10.7567/APEX.10.012201

Refractive index sensing using ultrasonically crushed polymer optical ﬁbers
Shumpei Shimada1, Heeyoung Lee1, Makoto Shizuka1, Hiroki Tanaka1, Neisei Hayashi2, Yukihiro Matsumoto3,
Yosuke Tanaka4, Hitoshi Nakamura5, Yosuke Mizuno1*, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan3
Department of Architecture and Civil Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
4
Institute of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
5
Department of Civil and Environmental Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received October 26, 2016; accepted November 7, 2016; published online November 24, 2016
We demonstrate power-based refractive index (RI) sensing using an ultrasonically crushed polymer optical ﬁber (POF). This structure can beeasily and cost-effectively fabricated within a short time (i.e., >1 s) without the need to employ external heat sources or chemicals. The only
requirement is to simply press a horn connected to an ultrasonic transducer against part of the POF. The RI dependence of the transmitted powershows linear trends in RI ranges of >1.32 to >1.36 [coefﬁcient: %62 dB/RIU (RI unit)] and >1.40 to >1.44 (coefﬁcient: %257 dB/RIU). The temperature dependence of the transmitted power is also investigated. © 2017 The Japan Society of Applied Physics

ptical ﬁber sensors have been vigorously studied
because of their ability to measure various physicalparameters, such as strain,1,2) temperature,1,2) pres3)
sure, acoustic impedance,4,5) humidity,6) reﬂectivity,7,8) andnuclear radiation.9) Among these applications, refractive index(RI) sensing using optical ﬁbers has been attracting considerable attention in biological and chemical research. Furthermore, numerous techniques have been developed in the past
several decades.10–13) RI sensing based on evanescent waves
generated in the tapered region of glass optical ﬁbers12,13)
is one of the most cost-eﬀective techniques having highsensitivity. However, glass optical ﬁber tapers are fragile andneed to be fabricated and handled with care. One promising
way to address this concern is to exploit polymer optical ﬁber(POF) tapers,14–20) which exhibit much greater ﬂexibility.
Various POF tapering methods have been developed todate. Two widely used methods are based on a heat-and-pull
technique with an external heat source14–17) and a chemical
etching technique.18) However, employing external heatsources (e.g., ﬂame,14) compact furnaces,15,16) or a solder
gun17)) and chemicals is unsafe and inconvenient. A POF
tapering technique for converting the propagating light energy
into heat without using an external heat source has been developed.19) However, the required high-power light injection
may cause burning at the POF ends21) and=or what is called
the fuse eﬀect.22,23) RI sensing using V-shaped POFs has
also been reported.20) However, their measurement accuracy
is not suﬃciently high because of their structural instability.
To the best of our knowledge, this is the ﬁrst work todemonstrate RI sensing using an ultrasonically crushed POF.
This structure can be easily fabricated without the need to
employ external heat sources or chemicals. A horn connected
to an ultrasonic transducer is pressed against part of the
POF for only a short period. First, we clarify that the RI
dependence of the transmitted power shows nonmonotonic
behavior, which indicates that this RI sensor can be directly
used only in limited RI ranges. For instance, the dependence
coeﬃcients are −62 dB=RIU (RI unit) and −257 dB=RIU in
RI ranges of ∼1.32 to ∼1.36 and ∼1.40 to ∼1.44, respectively. We then ﬁnd that the transmitted power shows anegative dependence on temperature with a coeﬃcient of−0.094 dB=°C when the RI is ﬁxed at 1.33, thereby leading
to the feasibility of temperature compensation in RI sensing.

O

Fig. 1. Processing procedure for the partially crushed polymer opticalﬁber. FG: function generator.

The POF used in the experiment was a perﬂuorinatedgraded-index (PFGI) POF24,25) with a core diameter of 50 µm,
a cladding diameter of 70 µm, an overcladding diameterof 490 µm, and a propagation loss of ∼250 dB=km at 1550
nm. The RIs of the core center, cladding, and overcladding
at 1550 nm were 1.356, 1.348, and 1.590, respectively.
The core and cladding layers consisted of identical material[i.e., poly(perﬂuorobutenylvinyl) ether with diﬀerent dopant
concentrations]. Their boundary was not visible. In contrast,
the overcladding layer was composed of polycarbonate.
Its boundary with the cladding layer was observable witha microscope. PFGI-POFs are known to absorb much lesswater than the standard poly(methyl methacrylate)-based
POFs,14,16) which is desirable for liquid RI sensing.26)
Figure 1 illustrates the POF processing procedure. A horn
(diameter: 6 mm) connected to a Langevin-type ultrasonictransducer with a resonance frequency of ∼18.7 kHz waspressed for 1 s against the midpoint of the 40-cm-long POF on
an anvil (made of stainless steel) with a 10 N preload.27) Notethat the output voltage from a function generator was ampliﬁed and applied to the piezoelectric stack of the transducer.
The vibration velocity was measured by Doppler velocimetry
to be 2.3 m=s. A 6-mm-long region of the POF was thencrushed. The POF was ﬁrst partially melted using this ultrasonic method and then solidiﬁed after distortion. The processes resulted in stable fabrication. Stable fabrication of thecrushed structure was diﬃcult because simple pressing without
ultrasonic oscillations was susceptible to incomplete plastic

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Appl. Phys. Express 10, 012201 (2017)

S. Shimada et al.

Fig. 2. Experimental setup for RI sensing using the crushed POF. MMF:
multimode ﬁber, OSA: optical spectrum analyzer, and SMF: single-mode
ﬁber.

deformation. The crushed region was bent around its midpoint
for ease of handling. It was also used as a sensing head.
Figure 2 shows an experimental setup for RI sensing using
the crushed POF. The output light from a laser at 1550 nm(power: 10 dBm; bandwidth: ∼1 MHz) was guided througha 1-m-long silica single-mode ﬁber (SMF) and injected
into the POF. The transmitted light was then guided to an
optical spectrum analyzer (OSA) via a 3-m-long silica
multimode ﬁber with a core diameter of 50 µm to suppress
the optical coupling loss at the boundary between the POF
and the silica ﬁber. Its spectral peak power was precisely
measured using 100× averaging. Both POF ends wereconnected to the silica ﬁbers by butt-coupling via ferruleconnector=subscriber connector adaptors.25)
We ﬁrst investigated the RI dependence of the transmitted
power when the crushed region was immersed in a sucrose
solution at 25 °C, the RI of which was varied from 1.318
to 1.437 by controlling the concentration in the range from0 to 65%.28) Subsequently, the temperature dependence ofthe transmitted power was investigated in the range from10 to 35 °C when the ambient RI of the crushed region was
ﬁxed at 1.333 (concentration: 10%).
Figures 3(a)–3(d) show micrographs of the crushed POF.
Figures 3(a) and 3(b) present side views around the boundary
between the crushed and uncrushed regions and around the
middle of the crushed region, respectively. The crushed region
had a height of ∼270 µm, which was ∼1.8 times smaller
than the outer diameter (490 µm) of the uncrushed region.
Figures 3(c) and 3(d) show top views around the boundary
and around the midpoint, respectively. The width of thecrushed region was ∼850 µm, which was ∼1.7 times larger
than the initial outer diameter (490 µm). Figure 3(e) presents a
photograph of the crushed POF after bending. The bending
angle around the midpoint of the crushed region was ∼35°.
Figure 4(a) shows the measured RI dependence of thetransmitted power at 25 °C. The vertical axis was normalized
such that the power at RI = 1.318 (sucrose concentration =0%) became 0 dB. Note that the propagation loss in the
crushed region of the POF was ∼35 dB. The transmitted
power decreased, and the loss increased as the RI increasedto ∼1.36. However, it increased by ∼1 dB when the RI
increased to ∼1.37. Subsequently, the power decreased
monotonically with increasing RI. The dependence abruptly
became large when the RI was higher than ∼1.40. When
the RI was 1.333, 1.384, and 1.437, the measurement errors,deﬁned as the standard deviations of the power ﬂuctuations(the power was measured every 1 min for 1 h) were ±0.30,
0.42, and 0.63 dB, respectively. This nonmonotonic behavior
seems to originate in part from a change in the phasemismatching condition among the modes involved in the

(a)

(b)

(c)

(d)

(e)
Fig. 3. Micrographs of the crushed polymer optical ﬁber: (a, b) side views
and (c, d) top views; (a, c) around the boundary between the crushed anduncrushed regions and (b, d) around the middle of the crushed region; and(e) photograph of the crushed region bent around its midpoint.

(a)

(b)

(c)

(d)

Fig. 4. Measured relative transmitted power dependence on the RI. Panels(b) and (c) show magniﬁed views of (a); (d) is the same measurement result
as (a) when an independently fabricated crushed POF was used. The RI
ranges are (a, d) 1.318–1.437, (b) 1.318–1.357, and (c) 1.404–1.437.

process when the external RI is varied, as observed in tapered
silica SMF.29) The situation should be more complicated,
not only because of the multimodal nature of the POF

012201-2© 2017 The Japan Society of Applied Physics

Appl. Phys. Express 10, 012201 (2017)

S. Shimada et al.

Fig. 5. Relative transmitted power dependence on temperature when the
RI was ﬁxed at 1.333.

but also because of the non-axisymmetrical structure of thecrushed POF. The transmitted power and the RI were not in
one-to-one correspondence with each other. Hence, the RI
range needed to be limited for practical sensor application.
The transmitted power directly provided the RI value with adependence coeﬃcient of −62 dB=RIU if the RI range
was limited to ∼1.32 to ∼1.36 [Fig. 4(b), magniﬁed view ofthis RI range]. In the same manner, the transmitted power
corresponded with the RI with a dependence coeﬃcient of−257 dB=RIU if the RI range was limited to ∼1.40 to ∼1.44[Fig. 4(c)]. Fairly comparing these absolute values with thosein previous reports measured under diﬀerent conditions and
POF structures was diﬃcult. However, the negative coefﬁcient of the transmitted power (or the positive coeﬃcient of
the loss) against the RI agreed with some of the previous
POF-taper-based results.14,19) No quantiﬁable inﬂuence on
the transmitted power was observed when the uncrushed
regions of the POF were immersed in the sucrose solutions.
Note that this nonmonotonic behavior was reproduced tosome extent when we used an independently fabricatedcrushed POF [see Fig. 4(d)], showing a higher repeatability
than other POF-based techniques we have reported;19,20)
further investigation of this point is required.
We ﬁnally set the ambient RI of the crushed region to1.333 (sucrose concentration: 10%), which was located near
the midpoint of the linear region shown in Fig. 4(b). We
then investigated the temperature dependence of the transmitted power in the range from 10 to 35 °C. The verticalaxis was normalized so that the power measured at 25 °C
became 0 dB. The inﬂuence of the temperature dependence ofthe sucrose solution was compensated.30) The transmitted
power decreased monotonically with increasing temperature(Fig. 5). This behavior was the same as that of the bentPOF-based sensors.31) The temperature dependence coeﬃcient of the transmitted power was −0.094 dB=°C when
we applied a roughly linear ﬁt to the data in this range. The
temperature-dependent error in RI sensing can be compensated in principle once this one-to-one correspondence
between the RI and the temperature is known.
In conclusion, we demonstrated RI sensing by exploiting
an ultrasonically crushed POF, the structure of which can beeasily fabricated without the need to employ external heatsources or chemicals. The only requirement is to simply press
a horn connected to an ultrasonic transducer against part of
the POF for a short time (e.g., ∼1 s). The transmitted power
was ﬁrst experimentally shown to have a nonmonotonic
dependence on the RI with moderate repeatability. Thisobservation indicated that using this sensor directly in theentire RI range was diﬃcult. However, the transmitted power
and RI in limited RI ranges were in a one-to-one correspondence with dependence coeﬃcients of −62 dB=RIU (RI
from ∼1.32 to ∼1.36) and −257 dB=RIU (RI from ∼1.40
to ∼1.44). A negative dependence of the transmitted poweron the temperature (coeﬃcient: −0.094 dB=°C) was also
clariﬁed when the RI was ﬁxed at 1.33, which leads tothe possibility of temperature compensation in RI sensing.
Thus, we anticipate that this unique RI sensing based on
the ultrasonically crushed POF will be of great use in
implementing stable and cost-eﬀective ﬁber-optic RI sensors
for future biological and chemical applications.
Acknowledgments This work was supported by the MLIT Construction
Technology Research and Development Subsidy Program; by JSPS KAKENHI
Grant Numbers 25709032, 26630180, and 25007652; and by research grants fromthe Japan Gas Association, the ESPEC Foundation for Global Environment
Research and Technology, and the Association for Disaster Prevention Research.

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012201-3© 2017 The Japan Society of Applied Physics